Nuts
From what I remember form high school chemistry and physics, it's very rare for any form of combustion to be complete -- especially in situations where there are large fans present like inside a jet engine. The small percentage of the (vast quantity of) fuel that is not burned will pass out with the exhaust gasses, leaving an effect similar to a crop duster at 40000ft.
No big deal, it's biodegradable, and will be spread over a very large area at low concentrations, it's never been a problem with conventional jet fuel... No big deal until someone says the phrase nut allergy...
Does this mean that people will be dropping down dead from nut alergies everywhere?